The purpose of this ethnographic study was to explore the development of belief systems as related to racial and ethnic identities of preservice teachers as they crossed cultural borders into science teaching. Data were collected throughout a yearlong teacher preparation program to learn how early life experiences and racial and ethnic identities of preservice teachers influenced both their beliefs about diversity in science classrooms and science teaching pedagogy. Case studies of three preservice teachers from diverse racial and ethnic background are presented: Asian American, African American, and Rural Appalachian. Using Bank's ethnicity typology, findings suggest that racial and ethnic identity, developed in early life experiences of preservice teachers, provided clarity on the rigidity of their beliefs about diversity and how they view science teaching. By learning about the border crossing experiences of preservice teachers in relation to their beliefs about diversity as related to racial and ethnic identities, the researchers hoped to provide insight on preparing preservice teachers for the challenges of working in diverse classrooms. © 2004 Wiley Periodicals, Inc. J Res Sci Teach 41: 119–141, 2004